Notification Error Subcodes

This 1-octet unsigned integer provides more specific information
about the nature of the reported error. Each Error Code may
have one or more Error Subcodes associated with it. If no
appropriate Error Subcode is defined, then a zero (Unspecific)
value is used for the Error Subcode field, and the O-bit must be
zero (i.e. the connection will be closed). The notation used in
the error description below is: MC = Must Close connection =O-bit
is zero; CC = Can Close connection = O-bit might be zero.